An IA needs to sign a 337 to certify the finished work conforms to it, but how do you get the 337 checked and approved by an FAA FSDO before the work is commenced?
AIUI there are only 2 routes: a prior approval via the FSDO and sending the IA-signed 337 after the job is done direct to AFS-750 for filing (which is the gold plated method, with no comeback), or skipping the FSDO pre-approval step and sending the IA-signed 337 after the job is done direct to AFS-750 for filing (which could come back and bite you one day, but not likely to if a DER was used to generate the Approved Data, or the a.d. is a directly applicable STC).
This was explained to me by a senior FAA inspector. Of course he may have been laying it on a bit thick, too...